exfat: fix incorrect loading of i_blocks for large files

When calculating i_blocks, there was a mistake that was masked with a
32-bit variable. So i_blocks for files larger than 4 GiB had incorrect
values. Mask with a 64-bit variable instead of 32-bit one.

Fixes: 5f2aa075 ("exfat: add inode operations")
Cc: stable@vger.kernel.org # v5.7+
Reported-by: Ganapathi Kamath <hgkamath@hotmail.com>
Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>